Under operating conditions, a turbomachine component, for example, a gas turbine airfoil, is subjected to a variety of forces. Some forces are dependent on rotor speed, e.g. centrifugal force, resulting in a steady state or slowly varying strain (change in dimension, e.g., stretching or shortening) of the airfoil. Others result in a more dynamically varying strain, i.e., commonly referred to as vibratory strain, and airfoil vibration, e.g., forced vibration (resonance or buffeting) and aero elastic instability (flutter). The magnitudes of the forces and resulting strains depend on the engine operating conditions and the aircraft structural and aerodynamic properties.
To prevent damage to the airfoil, the magnitudes of the steady state and vibratory strains must not exceed the structural capabilities (limits) of the airfoil. In order to keep the vibratory strain of the airfoil within limits, the engine is often operated at lower than optimum conditions, resulting in a reduced engine operating efficiency.
Various approaches exist for reducing airfoil vibration. Some of these approaches involve stiffening the structure of the airfoil. The effect of stiffening is to adjust the resonant frequency of the airfoil to a value that is different from that of the vibratory force. Increased stiffness helps to prevent flutter-type vibratory strain. For example, a more rigidly constructed airfoil results in less vibration. However, a more rigid airfoil is often heavier (with associated disadvantages) and the optimum degree of rigidity is often not precisely known at the time that the airfoil is initially designed. Another approach makes use of a shroud, disposed at a midspan point on the airfoil. A midspan shroud has the effect of stiffening the airfoil. In addition, the shrouds interact with one another to reduce vibration of multiple adjacent blades. However, a midspan shroud tends to obstruct the airflow and thereby reduce turbomachine efficiency.
Passive vibration damping is another approach for reducing the magnitude of airfoil vibration. Passive vibration damping is a form of structural damping that involves the dissipation of energy. One approach for passive damping employs sliding friction devices, such as those employed under blade platforms. This approach relies on friction to dampen vibratory motion. However, most blade vibratory motion occurs above the platform, for which under-platform devices have limited effectiveness.
An active vibration control scheme has been proposed by Acton et al. in U.S. Pat. No. 4,967,550. The scheme uses a control system with actuators to counter blade vibration. Acton et al. disclose that two categories of actuators involving direct contact with the blades: "(i) electromagnetically actuated shakers attached to the blades for introducing forces in the blades, and (ii) piezoelectric or magnetostrictive means internal of the blades to deform them by changing the relevant characteristics of such, for examples embedded piezoelectric crystals which could distort the blade and thereby affect the local structural properties of the blades, e.g. by increasing the structural damping." Piezoelectric materials convert electrical energy to mechanical energy, and visa versa. Unlike passive methods, an active control system, sometimes referred to as a feedback system, is complex, requiring sensors, signal processing circuits, actuators, and a power supply. Embedding piezoelectric crystals in the blade requires a complex fabrication process. The combination of an active control system and embedded piezoelectric crystals is not practical in terms of cost and complexity.